Aperture mask coating to prevent cathode poisoning



May 12 1959 J. L.. SHELDON 2,886,730

APERTURE MASK coA'rING To PREVENT CATHODE PolsoNING Filed Feb. 25, 1957United lStates Patent@ APERTURE Mask` CoAnNG r'ro PREVENT cArHoDEPoisoNiNG Application February 25, 1957, Serial No. 642,074 i 3 Claims.(Cl. 313-86) This invention relates to an improved perforate mask forassembly in a cathode ray tube of the type used in multicolor televisionreceivers and to a cathode ray tube embodying such mask.

Certain types of multicolor television systems operate on the generalprinciple of emitting a stream of electrons from one or more electronguns mounted in the neck of a cathode ray tube and directing such streamof electrons through a perforate mask onto a viewing screen composedessentially of a phosphor pattern deposited on a face plate or panelforming the opposite end of the tube. The perforate mask, for examplethe present tube component commonly referred to as a shadow or aperturemask, is positioned transversely of the electron path intermediate theelectron gun and screen to intercept all electrons strik- ?ing the maskand thereby form a plurality of electron beams passing through theapertures or perforations in the mask to the pattern of phosphors ontheface of the tube.` Multicolor television systems of this sort aredescribed in detail in the patent literature, such as United StatesPatent #2,6217 34 to Harold F. Law.

The apertures in such a mask are small and closely spaced, and must beformed with an extreme degree of accuracy to produce a clear,Well-defined picture. Thus specifications of a typical mask call forapertures mils in diameter and uniformly spaced 28 mils center to centerand a mask thickness of 55 mils, with the aperture dimensions being heldwithin a tolerance of r0.5 mil. The inherent difficulties in producingand handling such a mask are quite readily apparent. Furthermore it isfre quently necessary to sag or otherwise form such a mask to a definitecurvature. p

Etched metal masks have been proposed, but their use requires a greatdeal of care to avoid warpage, bending, or other'physical damage duringproduction and subsequent handling. lt has been recognized `that a maskcomposed of a ceramic material such as glass should be highly desirable,but, until 'discovery of the chemical method fof `machining t glass..described in United- States Patent#2,628,l60.issued toS. D. Stookey, nosatisfactory method existed for producing such masks. It was naturallyassumed that, if a mask of glass or other ceramic could besatisfactorily produced, its use in a tube should present no particularproblem since glass is used in construction of the tube envelope itselfand other ceramics are widely used in electronic tubes.

Once glass masks were successfully made by the Stookey method, it wasquite unexpectedly found that tubes embodying such masks in theirassembly tended to fail shortly after being placed on life test.Extensive investigations into these tube failures indicated that thefailure was due to a foreign material poisoning the cathode of theelectron gun during its operation and that this foreign material was insome manner coming from the glass mask. It is then a primary purpose ofthe present invention to provide chemically perforated glass masks freefrom such prior deficiencies and capable of being used in cathode raytubes for absorption of electrons.

2,886,730 Patented I May `1 ice 2 t A t By way of illustration thepresent invention is described with reference to the accompanyingdrawing in which;" i

Fig. l is a schematic representation, largelyinsection,ofacathoderaytube and,`

Fig. 2 is an enlarged fragmentary view in section bf a perforate mask inaccordance with the invention and,

Fig. 3 is an enlarged fragmentary view in section, simi-` lar to Fig. 2,illustrating a modification of the invention;

In accordance with the present inventiom satisfactory glass aperturemasks areproduced `by providing tlieelbc-` tron intercepting surface ofa chemically perforated 'glass mask with a coating composed of at leastone Alay'ero'f metal. By electron intercepting surface I meanithe glasssurface facing, and therefore exposed to, the electron emission gun andhence to the stream of electrons'emitted therefrom.

The cathode ray tube, schematically represented 4infFig. 1 anddesignated by the numeral 10, has an electron gun 12 mounted in the tubeneck 14 and a viewing screen 16 composed of a phosphor coating 18, ofsuitable composie` tion and pattern for multicolortelevising, applied tothe inside surface of transparent face plate or panel 20 which forms theopposite, enlarged end ofthetube. The phosphor coating is applied in anyknown manner ina dot; stripe or other type of pattern suitable for thepurpose. Mounted intermediate electron gun 12 and screen 16, `by anysuitable conventional means not specifically shown, is perforate glassmask 22, having a metal coating `I24 on the side of the mask facing gun12, that is the electron intercepting surface of the mask. 1

In Fig. 2 a fragment of mask 22 is shown greatly exagl-i gerated tobetter illustrate chemically perforatedapertures 26 which are too smalland too numerous'to be satisfactorily shown in a full view suchas Fig`l, andto better illustrate, as well, metal coatingv24` In accordancewith the exemplary mask specifications referred to earlier, the diameterof apertures 26 is 10 mils and they are spaced 28 mils apart center tocenter. For purposes ofillustra-` tion mask 22 is shown flat andapertures26, as sho'tt/n,` are' formed normal to the electronintercepting surface;y VAs a practical matter mask 22 will frequentlyhave imparted to it a definite curvature and apertures 26 maybe-'formedat angles up to 25 or so from normal dependingonthe location andgeometry of the tube components. The an-A gle of each aperture will besuch as to provide alignment of electron gun, apertures and screenelements forkp'ropler screen scanning. "i i In a preferred embodiment ofmy inventionfthe lec tron intercepting surface of mask 22 is coated witha layer of metal 24 of sufficient thickness that electrons-'emitted bygun 12 are absorbed by the metalland do not perle-i trate to the glassto any appreciable extent.` f Theiexact effect of electron bombardment"on chemically perforated glass has not been positively"ascertained. Itis'l m'y `belief however that absorption of electrons by this particulartype of glass results in a reaction within the glass accompanied byevolution of a gas which poisons the electron gun cathode, and there isexperimental evidence indicating that the offending gas is oxygen. Inany event it has been found that, if the glass is shielded by :anabsorbing layer of metal, poisoning of the cathode is either eliminatedor so minimized that normal tube life can be expected.

Various methods of applying metal to glass surfaces are available andmay be used in coating glass masks. The process of thermal evaporationis a convenient one since it is well developed commercially to providean adherent metal coating of controlled and adequate thickness on aglass surface. When this coating method is employed I prefer aluminum asthe coating metal since it is satisfactory for electron interception,and thermal evaporation of it is an established and well-understoodprocess in the manufacture of cathode ray tubes. However other inertmetals such as iron, nickel, and alloys of these metals may also besatisfactorily used and in some instances may even be preferable becauseof greater density. Thermally evaporated aluminum films heretoforeemployed in cathode ray tube production have been deposited as aconducting layer in conjunction with the phosphor screen on the tubeface plate. Such a conducting film extends to an electrical contact orterminal through the tube wall and serves as a metallic conductor todrain electrons from the screen and return them to the power supply. Thethickness of such conducting aluminum films is generally on the order of3000 to 5000 Angstrom units and is minimized to avoid producing anysubstantial absorption effect on the stream of electrons directed at thescreen.

The thickness of metal required on the glass mask for electronabsorption purposes is of considerably greater magnitude and dependsprimarily on the voltage drop from the electron gun to the tube screen,that is the velocity of the electrons to be intercepted, and on theeffective density of the metal coating employed, which in turn dependson the particular metal used. By way of specific illustrative example, aglass mask, designed for use in a cathode ray tube to operate at a ratedvoltage of 25 kilovolts, was provided witll an aluminum film having athickness of 75,000 Angstrom units and the resulting tube found toperform satisfactorily during a life test of 1000 hours which isconsidered to indicate a satisfactory tube for commercial use. On thebasis of theory an aluminum coating 55,000 Angstrom units in thicknessshould be adequate but in order to provide an adequate safety factor Iprefer to employ a thickness of 75,000 to 125,000 Angstrom units. Whilegreater thicknesses could of course be used, they appear to beunnecessary and uneconomical.

The thickness required will vary directly with the square of the voltageand inversely with density. Thus, if a lower operating voltage than 25kilovolts were being used, the minimum thickness might becorrespondingly reduced. Likewise with another metal having, forexample, an effective density, as deposited, three times that ofaluminum, the minimum thickness would be one third or about 25,000Angstrom units.

An alternative form of the invention is illustrated in Fig. 3 whereinthe electron intercepting surface of mask 22 is coated with two layersof metal 28 and 30. Metal layer 28 is relatively thin and in directadherent contact with the surface of mask 22. The metal employed shouldbe a very chemically active metal, such as barium, which will act in themanner of a getter to immediately react with any oxygen or other cathodepoisoning material evolved from the glass as a result of electronabsorption. Other active metals, or getters, such as magnesium mightalso be employed, and any metal capable of spontaneously reacting withor absorbing cathode poisoning agents such as oxygen are contemplatedfor the present purpose. Metal layer 30, also relatively thin, iscomposed of an inactive or inert metal and serves to protect the activemetal layer 28 from atmospheric gases with which the mask is in contactwhile the tube is being assembled and prior to the conventional bakeoutprocess. Metal layer 30, then, may be composed of an inert metal such asaluminum, iron, nickel, or the like. The duo-layered metal coating 28-30is preferably applied to mask 22 by means of a two-step thermalevaporation process carried out in a suitable vacuum chamber. Thus, forexample mask 22 might be mounted in a conventional thermal evaporationapparatus with suitable amounts of metallic barium and metallic aluminumpositioned for thermal evaporation. The barium would be rst deposited on the mask surface followed by deposition of a protective aluminumcoating.

While the overall thickness of the double metal coating required in thisembodiment of the invention will vary somewhat depending on tubeoperating conditions, it will, generally speak, be much thinner than anelectron absorbing type of film such as described earlier. The duo-layercoating is of course not intended to absorb electrons. The bariumportion of the film then need be only of sufiicient thickness to reactwith gases evolved, and the inert protective film need only be ofsufficient thickness to provide an effective barrier between the activemetal and the atmosphere. It may correspond to the conducting filmsheretofore applied over the phosphor screen of a cathode ray tube.

While the invention has been described essentially with relation to whatis currently known as a shadow mask, it will be appreciated that it isnot restricted to a mask having the specific size and pattern ofperforations in such mask but is equally applicable to other types ofperfo-I-` rate masks as well regardless of perforation pattern ori size.

What is claimed is:

1. A perforate mask for assembly in a cathode ray tube]` to selectivelytransmit electron beams which comprises a perforated glass sheet havingon its electron intercepting surface a coating composed of a chemicallyactive metal comprising a getter which is, in turn, covered and aprotective layer of inactive metal.

2. A mask in accordance with claim l wherein the chemically active metalis barium and the inactive metal is aluminum.

3. For use in multicolor television reception, a cathode ray tube havingan electron emission source, an electron sensitive screen, and aperforate, electron-absorbing mask mounted intermediate said electronsource and screen, said mask comprising a perforated sheet of glasshaving on its electron intercepting surface a coating composed of achemically active metal comprising a getter which is, in

turn, covered with a protective layer of inactive metal.

Dorgelo Mar. 16, 1948l Laiferty Ian. 8, 1957 UNITED STATES PATENT OEETCECERTIFlCATE OF CORRECTION May l2 1959 ars in the .pri

t the said Letters fied that error appe n and tha atent requiringcorrectio ted below.

lt is herebf certi of the above rmmoered p Patent should read as correoColumn A, d speaking line Al, for ncovered and rea Signed and sealedthiS` 15th day o September l959 SEAL) nest: KARL H MLM ROBERT C. WATSONCommissioner of Patents Attesting OHcer

1. A PERFORATE MASK FOR ASSEMBLY IN A CATHODE RAY TUBE TO SELECTIVELYTRANSMIT ELECTRON BEAMS WHICH COMPRISES A PERFORATED GLASS SHEET HAVINGON ITS ELECTRON INTERCEPTING SURFACE A COATING COMPOSED OF A CHEMICALLYACTIVE METAL COMPRISING A GETTER WHICH IS, IN TURN, COVERED AND APROTECTIVE LAYER OF INACTIVE METAL.